Element　doping　has　demonstrated　an　effective　strategy　for　enhancing　photocatalytic　performance　of　carbon　nitride　(CN).　Herein,　we　reported　that　oxygen-doped　CN　(OCN)　was　successfully　prepared　by　molten　urea　as　the　only　raw　precursor,　in　which　thermal　copolymerization　of　urea　and　in　situ　formed　biuret　can　effectively　incorporate　oxygen　in　the　CN　network.　The　introduction　of　oxygen　into　CN　network　greatly　improved　the　visible-light　absorption　capability,　increased　the　charge　carrier　mobility　and　inhibited　the　electron–hole　pairs　recombination.　In　addition,　the　incorporation　of　the　elemental　oxygen　with　strong　electronegativity　into　CN　matrix　afforded　OCN　with　more　structural　base　sites,　which　benefited　for　the　adsorption　of　CH3SH.　As　expected,　OCN　exhibited　an　enhanced　photocatalytic　performance　toward　CH3SH　degradation　and　mineralization　under　visible-light　irradiation.　This　study　demonstrates　that　a　simple　precursor　pretreatment　strategy　can　achieve　the　incorporation　of　beneficial　foreign　atoms　into　the　CN　framework,　which　provides　some　guidance　for　the　design　and　synthesis　of　high-efficiency　CN　photocatalyst　with　peculiar　feature.　Graphical　abstract:　O-doped　carbon　nitride　prepared　by　thermal　polymerization　of　molten　urea　exhibited　an　enhanced　visible-light-driven　photocatalytic　performance　for　CH3SH　degradation.　[Figure　not　available:　see　fulltext.]　©　2022,　The　Author(s),　under　exclusive　licence　to　Springer　Nature　B.V.